Cancerous cells release growth factors such as VEGF to induce growth of new blood vessels in their microenvironment. Neovascularization provides oxygen and nutrients to the growing tumor, while removing any waste products and providing an efficient route for the spread of tumor cells to other organ systems. In the absence of angiogenesis, tumors cannot develop more than 3-4 mm in size;hence, angiogenesis has become a therapeutic target. The vertebrate vasculature and Drosophila tracheal system share similar mechanisms of branch sprouting and tube formation, and therefore the tractable Drosophila model system will provide much needed insight into capillary lumenization during angiogenesis. Ultrastructural studies have shown that branch connection points are frequently comprised of "seamless" tubes. Seamless tubes are unicellular tubes that appear doughnut like in cross-section. Endothelial cells in culture form seamless tubes by the fusion of intracellular vacuoles before eventually connecting with other endothelial cells to generate larger tubes with intercellular seams. These observations have led to a "cell hollowing" model of seamless tube formation in which membrane vesicles or vacuoles are targeted to the center of the cell, where they coalesce and fuse to generate an internal apical membrane. This proposal seeks to define the genetic, molecular, and cell biological mechanisms that underlie formation of seamless tubes, taking a two-pronged approach to address this question in the Drosophila tracheal system. First, the process of seamless tube formation will be elucidated by studying mutants identified in a forward genetic screen that exhibit defects in seamless tubes. The second prong of the approach entails live cell imaging studies in which well-characterized and fluorescently-tagged membrane trafficking components are utilized to visualize seamless tube formation in vivo in real time. DIRECT RELEVANCE TO PUBLIC HEALTH: The induction of new blood vessels Is critical for many aspects of growth and development, but particularly important during wound healing and tumorigenesis. This proposal aims to describe the mechanism by which a cell can transform into a tube (such as a blood vessel) using the Drosophila trachea as a model system.